The Declaration of Helsinki, the most creditable ethical guideline for medical research on human subjects, has been updated 8 times since its establishment and the last revision was in 2008. Researchers, medical research subjects, authors, members of ethics committees, and editors of medical journals must be informed of the tenets of the Helsinki declaration in order to improve achievements of medical research and respect the rights of participants in medical research. In this study, the evolution of the Helsinki declaration is examined, and the provisions of the last version are compared with the previous version (2004). Finally, we made a practical interpretation of the last version, and discussed its differences from previous version. There are seven new paragraphs in the last version of the declaration. Four new paragraphs concern informed consent in areas of its being written and voluntary, renewed consent for using human materials, participants' right to revoke the consent, and exceptions from informed consent requirements. The other three new paragraphs focus on the research subjects' right to being informed of the declarations' provisions, vulnerable groups gaining benefit from research, and registration of randomized clinical trials. It is important for researchers to recognize international guidelines such as the Declaration of Helsinki, because it enables them to use correct scientific and ethical standards in medical research. In the last version of the Helsinki Declaration more emphasis was placed on informed consent and vulnerable groups than previous versions.